The present invention relates to MIMO (multiple-input multiple-output) devices and communications, and more particularly to a method of adapting the number of transmitting antennas in a MIMO wireless link.
MIMO represents an advance in wireless communication. MIMO employs multiple antennas at the transmitting and receiving ends of a wireless link to improve the data transmission rate while holding radio bandwidth and power constant.
A MIMO transmitter transmits an outgoing signal using multiple antennas by demultiplexing the outgoing signal into multiple sub-signals and transmitting the sub-signals from separate antennas. MIMO exploits the multiple signal propagation paths to increase throughput and reduce bit error rates. Each sub-signal reflects off the local environment along its associated signal propagation paths. The spatial richness of the local environment is a function of the uniqueness and distinctness among the different associated signal propagation paths. While multiple signal propagation paths cause interference and fading in conventional radios, MIMO uses these multiple signal propagation paths to carry more information than conventional radio transmissions. Using MIMO techniques it is possible to approximately, linearly increase the rate of transmission, depending on the richness of the local environment.
FIG. 1 illustrates a basic MIMO wireless link 10, where the transmitter 20 has Mmax transmitting antennas 21 (21-1 . . . 21-m), and the receiving station 30 has N receiving antenna 31 (31-1 . . . 31-n), the number of transmitters active at a given moment is M, such that M<=Mmax. A scattering environment 50 with some degree of spatial richness, or statistical independence of fading coefficients, exists between the transmitter and receiver. The H matrix represents the channel connection characteristics (or impulse response) between the transmitting and receiving antennas, 21 and 31, respectively.
In a traditional Frequency Division Duplex (FDD) system, the MIMO receiver must determine M and feed this back to the transmitter on a separate low rate channel, such as mode selection link 40, as shown in FIG. 1. In a Time Division Duplex (TDD) system, no feedback link is necessary, as each end of the communications link can determine the transmission parameters independently.
In the situation where both the transmitter and receiver are stationary, the estimated channel characteristic of the MIMO system remains relatively stable, as does the optimal number of transmission antennas. However, where the transmitter, receiver, or objects in the environment are mobile, the actual channel characteristics of the connection and the spatial richness of the environment can change in response to movement. As the spatial richness of the environment changes, it becomes beneficial to vary the number of active antennas in the MIMO system to optimize the throughput of the wireless transmission. Varying the number of antennas in the MIMO system can offer various benefits including improved transmission rates, reduced interference among sub-signals, lower latency, and reduced power consumption.
For example, as the spatial richness in an environment increases, it may be beneficial to harness the increased variation in the multi-path signal propagation by increasing the number of active antennas. Alternatively, as the spatial richness decreases it may be beneficial to reduce the number of active antennas to avoid potential interference due to the limited signal path variations, and reduce power consumption by using fewer active transmitting antennas, which would otherwise cause interference. Therefore, there exists a need for a method to dynamically alter the number of transmitting antennas in response to, for example, changes in spatial richness.